In addition to generating and editing the MODFLOW input files, GMS can also be used for post-processing the solution files computed by MODFLOW. The solution files computed by MODFLOW include head, drawdown, and CCF (Cell-to-Cell Flow) files. By default, the head and CCF files are automatically generated. The output options are controlled in the MODFLOW Output Control dialog.

No-Flow and Dry Cells

When a MODFLOW simulation is solved, MODFLOW writes out a head or drawdown value for every cell of the finite difference grid to the solution files. However, some of the cells are either outside the problem domain or they have gone dry during the course of the simulation. These cells are flagged by MODFLOW in the output file by writing special values for the cells. The value assigned to inactive cells is the No flow head value specified in the Global Options/Basic Package dialog. The value assigned to cells which have gone dry is the Head assigned to dry cells value defined in the BCF, LPF or HUF package dialogs. If the MODFLOW data are in memory when the solution is read in, GMS will automatically use the No flow head and Head assigned to dry cells values to define active/inactive cells for post-processing.

Cell Summary Text File

When a MODFLOW solution is imported into GMS there is an option to generate a "cell summary" text file. This file lists the time, stress period, time step and the number of active, inactive, dry and flooded cells. If the file exists then it will be added to the MODFLOW solution in the Project Explorer and the file can be opened in a text editor from GMS. The file is in *.csv format so that spreadsheet programs can easily read and format the file into columns. Two examples are shown below.

The active cells include all cells with an IBOUND value not equal to 0 and cells that have had the IBOUND change to nonzero during the model run (this can occur if the rewetting option is used in the flow package). The inactive cells include cells with IBOUND=0 as well as cells that are set to IBOUND=0 while the simulation was running (Some versions of MODFLOW will do this to dry cells; MODFLOW-NWT does not inactivate dry cells). The dry cells include cells that have the value of HDRY assigned to them or cells where the head is below the bottom elevation of the cell (Note with MODFLOW-NWT a dry cell can still be active.). The flooded cells include cells where (1) the head is above the top elevation of the cell, (2) the cell is not in a layer with a confined layer type defined in the flow package, and (3) the cell is the highest active cell in the vertical column of cells.

Layer Contours

In most cases, the best way to display computed head and drawdown is with layer contours. Layer contours are generated by selecting the Contours option in the 3D Grid tab of the Display Options dialog. This option is automatically turned on whenever a MODFLOW solution is read into GMS. The Mini-Grid Toolbar, which is part of the Tool Palettes, can be used to switch between layers.

Viewing Computed Fluxes

The CCF file that is part of the MODFLOW solution contains useful information about the computed flux rates between the aquifer and external sources and sinks. A special set of tools are provided for viewing computed fluxes.

Summation of Fluxes on Arcs and Polygons

GMS writes the necessary files so that MODFLOW will output the computed flow for boundary conditions created in the Map module. These computed flows can be viewed by selecting the appropriate feature object (river arc, etc) and then looking at the Status Bar. For an object with an assigned observed flow, the observed flow value and the residual error are displayed in addition to the computed flow. For objects without an observed flow, only the computed flow is shown.

This cannot be done for SFR stream arcs since the SFR package does not participate in the Observation Process.

Calibration Targets and Statistics

For objects with an observed flow, a calibration target can be plotted on the object. The calibration target provides a graphical representation of the calibration error. Calibration targets are described in section 14.5. The display of flow calibration targets is turned on by selecting the Display Options command in the Feature Objects menu when the local source/sink coverage is the active coverage.

Flow Budget for Selected Cells

If a MODFLOW model is built without using the conceptual model approach (directly from the grid), the computed fluxes corresponding to a user-defined set of cells can still be displayed. This is accomplished by selecting a set of cells and selecting the Flow Budget command from the Data menu in the 3D Grid module. This command brings up the Flow Budget dialog. This dialog lists the complete flow budget for the selected cells.

GMS also includes a Zone Budget tool to view the computed flows into (out of) user defined zones.

Vector Plots

If a CCF file has been imported as described above, a vector plot can be generated to illustrate the flow field computed by MODFLOW. The CCF file contains flows through each of the cell walls in the grid, i.e., the flow from each cell to each of its six surrounding cells.

Vectors are generated by right-clicking on the CCF file in the Project Explorer and selecting the CCF→Velocity Vectors command. The cell top and bottom elevations as well as the porosity are used when calculating the velocity vectors. Therefore, for GMS to compute the vectors, a MODFLOW simulation must exist in GMS. Although porosity is not an input to a MODFLOW model, the porosity can be specified per cell, using the cell properties dialog, whenever a MODFLOW simulation exists.

The vectors are computed by reading the CCF file to get the net flow through each grid cell face. Each component of the velocity vector is computed by dividing the flow through the cell face by the saturated area of the face multiplied by the porosity. For example, the I, J, K direction components of the vector are calculated as follows:

The saturated area may be less than the area of the cell face if the computed head is below the top elevation of the grid cell. The velocity vector is set to 0.0 in cells that are inactive, dry (the computed head is below the cell bottom elevation), or have inappropriate elevations (the top elevation is below the bottom elevation).

Prior to version 8.2.

To generate a vector dataset from the CCF file, right-click on the CCF dataset in the Project Explorer and select the Generate vectors command from the pop up menu. A flow vector is generated at each cell center by computing a vector sum of the flows through the six walls of the cell. The resulting vectors can be plotted by selecting the Vectors option in the 3D Grid Display Options dialog.

Viewing the Printed Output File

Two types of output are produced by MODFLOW: a printed output file and a set of solution files (head, drawdown, CCF). Before reading in the solution files, it is often useful to examine the printed output file. In some cases, MODFLOW may crash or not complete its run successfully. Determine if the run was completed successfully by viewing the printed output file. When viewing the file, check to make sure that a solution was output for all stress periods and time steps that were expected. In some cases MODFLOW will also output to the listing file a description of any problems which may have occurred.

GMS provides two ways to view the text files produced by MODFLOW and the other analysis codes:

When a solution is read into GMS, the text output files are placed into the data tree. Double-clicking on their file icons in the Project Explorer will bring up the text files in a text editor.

Any text file can be viewed by selecting the Edit File command in the File menu. A File Browser appears and the selected file is opened in a text editor.